View source for Parsec

{{short description|Unit of length in astronomy}}
{{Other uses}}
{{Use dmy dates|date=May 2020}}
{{Infobox unit
| image    = Stellarparallax parsec1.svg
| image_size = 200px
| caption  = A parsec is the distance from the Sun to an [[astronomical object]] that has a [[parallax]] angle of one [[Minute and second of arc#Symbols and abbreviations|arcsecond]] (not to scale)
| standard = astronomical units
| quantity = [[length]]/[[distance]]
| symbol   = pc
| units1   = [[metric system|metric]] ([[International System of Units|SI]]) units
| inunits1 = {{convert|1|pc|m|disp=out|sigfig=5|lk=on}} <br />{{nbsp|3}}≈{{convert|1|pc|Pm|disp=out|sigfig=2|abbr=off|lk=on}}
| units2   = [[Imperial units|imperial]]&nbsp;&nbsp;[[United States customary units|US]]&nbsp;units
| inunits2 = {{convert|1|pc|mi|disp=out|sigfig=5|lk=on}}
| units3   = [[Astronomical system of units|astronomical units]]
| inunits3 = {{convert|1|pc|au|disp=out|sigfig=6|lk=on}}<br />{{nbsp|3}}{{convert|1|pc|ly|disp=out|sigfig=6|lk=on}}
}}

The '''parsec''' (symbol: '''pc''') is a [[unit of length]] used to measure the large distances to [[astronomical object]]s outside the [[Solar System]], approximately equal to {{convert|1|pc|ly|2|abbr=off|lk=out|disp=out}} or {{convert|1|pc|AU|0|abbr=off|lk=out|disp=out}} (AU), i.e. {{convert|30.9|e12km|e12mi|abbr=off|lk=on}}.{{efn|name=trillion|One trillion here is [[long and short scales|short scale]], ie. 10<sup>12</sup> (one million million, or billion in long scale).}} The parsec unit is obtained by the use of [[parallax]] and [[trigonometry]], and is defined as the distance at which 1 AU [[subtended angle|subtends]] an angle of one [[arcsecond]]<ref>{{Cite web |title=Cosmic Distance Scales – The Milky Way |url=https://heasarc.gsfc.nasa.gov/docs/cosmic/milkyway_info.html |access-date=24 September 2014}}</ref> ({{sfrac|3600}} of a [[degree (angle)|degree]]). The nearest star, [[Proxima Centauri]], is about {{convert|1.3|pc|ly|abbr=off}} from the [[Sun]]: from that distance, the gap between the Earth and the Sun spans slightly less than one arcsecond.<ref>{{Cite conference |last=Benedict |first=G.&nbsp;F. |display-authors=etal |title=Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri | url = http://clyde.as.utexas.edu/SpAstNEW/Papers_in_pdf/%7BBen93%7DEarlyProx.pdf |pages=380–384 |access-date=11 July 2007 |book-title=Proceedings of the HST Calibration Workshop}}</ref> Most [[Naked-eye stars|stars visible to the naked eye]] are within a few hundred parsecs of the Sun, with the most distant at a few thousand parsecs, and the [[Andromeda Galaxy]] at over 700,000 parsecs.<ref>{{cite web |title=Farthest Stars |url=https://stardate.org/radio/program/2021-05-15 |website=[[StarDate]] |publisher=[[University of Texas at Austin]] |access-date=5 September 2021 |date=15 May 2021}}</ref>

The word ''parsec'' is a shortened form of ''a distance corresponding to a parallax of one second'', coined by the British astronomer [[Herbert Hall Turner]] in 1913.<ref name="dyson">{{Cite journal |last=Dyson |first=F.&nbsp;W. |author-link=Frank Watson Dyson |date=March 1913 |title= The distribution in space of the stars in Carrington's Circumpolar Catalogue |journal= [[Monthly Notices of the Royal Astronomical Society]] |volume=73 |issue=5 |page=342 <!-- the whole article is at pp.=334–345 but single page in the source that supports the content" has preference. Note that both OUP.com and Harvard.edu PDFs are truncated at p. 342 --> | bibcode=1913MNRAS..73..334D |doi=10.1093/mnras/73.5.334 |doi-access=free | quote= [''paragraph 14, page 342''] Taking the unit of distance R* to be that corresponding to a parallax of 1″·0 [… Footnote:] <br> * There is need for a name for this unit of distance. Mr. [[Carl Charlier|Charlier]] has suggested [[Sirius|Sirio]]meter, but if the violence to the Greek language can be overlooked, the word ''Astron'' might be adopted. Professor [[Herbert Hall Turner|Turner]] suggests ''Parsec'', which may be taken as an abbreviated form of "a distance corresponding to a parallax of one second".}}</ref> The unit was introduced to simplify the calculation of astronomical distances from raw observational data. Partly for this reason, it is the unit preferred in [[astronomy]] and [[astrophysics]], though in [[popular science]] texts and common usage the [[light-year]] remains prominent. Although parsecs are used for the shorter distances within the [[Milky Way]], multiples of parsecs are required for the larger scales in the universe, including [[kilo-|kilo]]<nowiki/>parsecs (kpc) for the more distant objects within and around the Milky Way, [[Mega-|mega]]<nowiki/>parsecs (Mpc) for mid-distance galaxies, and [[giga-|giga]]<nowiki/>parsecs (Gpc) for many [[quasar]]s and the most distant galaxies.

In August 2015, the [[International Astronomical Union]] (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent [[bolometric magnitude]] scale, mentioned an existing explicit definition of the parsec as exactly {{sfrac|{{Val|648000}}|{{pi}}}}&nbsp;au, or approximately {{Val|30856775814913673|}}<!-- if absurdly many digits are needed, let the full listing correspond to rounded meters -->&nbsp;metres, given the IAU 2012 exact definition of the astronomical unit in metres. This corresponds to the small-angle definition of the parsec found in many astronomical references.<ref>{{Cite book |title=Allen's Astrophysical Quantities |date=2000 |publisher=AIP Press / Springer |isbn=978-0387987460 |editor-last=Cox |editor-first=Arthur N. |edition=4th |location=New York |bibcode=2000asqu.book.....C}}</ref><ref>{{Cite book |last1=Binney |first1=James |title=Galactic Dynamics |last2=Tremaine |first2=Scott |date=2008 |publisher=Princeton University Press |isbn=978-0-691-13026-2 |edition=2nd |location=Princeton, NJ |bibcode=2008gady.book.....B}}</ref>

== History and derivation ==
{{See also|Stellar parallax}}
Imagining an elongated [[right triangle]] in space, where the shorter leg measures one au ([[astronomical unit]], the average [[Earth]]–[[Sun]] distance) and the [[subtended|subtended angle]] of the vertex opposite that leg measures one [[arcsecond]] ({{frac|3600}} of a degree), the parsec is defined as the length of the [[Trigonometry#Trigonometric_ratios|''adjacent'']] leg. The value of a parsec can be derived through the rules of [[trigonometry]]. The distance from Earth whereupon the radius of its solar orbit subtends one arcsecond.

One of the oldest methods used by astronomers to calculate the distance to a [[star]] is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is taken approximately half a year later, when the Earth is on the opposite side of the Sun.{{efn|name=orbit|Terrestrial observations of a star's position should be taken when the Earth is at the furthest points in its orbit from a line between the Sun and the star, in order to form a right angle at the Sun and a full au of separation as viewed from the star.}} The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the distant [[Vertex (geometry)#Of an angle|vertex]]. Then the distance to the star could be calculated using trigonometry.<ref name="NASAparallax">{{Cite web |title=Deriving the Parallax Formula |url=http://imagine.gsfc.nasa.gov/YBA/HTCas-size/parallax1-derive.html |last=[[High Energy Astrophysics Science Archive Research Center]] (HEASARC) |website=NASA's Imagine the Universe! |publisher=Astrophysics Science Division (ASD) at [[NASA]]'s [[Goddard Space Flight Center]] |access-date=26 November 2011}}</ref> The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer [[Friedrich Wilhelm Bessel]] in 1838, who used this approach to calculate the 3.5-parsec distance of [[61&nbsp;Cygni]].<ref>{{Cite journal |last=Bessel |first=F.&nbsp;W. |author-link=Friedrich Wilhelm Bessel |date=1838 |title=Bestimmung der Entfernung des 61sten Sterns des Schwans |trans-title=Determination of the distance of the 61st star of Cygnus |url=https://zenodo.org/record/1424605 |url-status= |journal=[[Astronomische Nachrichten]] |volume=16 |issue=5 |pages=65–96 |bibcode=1838AN.....16...65B |doi=10.1002/asna.18390160502 |archive-url= |archive-date=}}</ref>

[[Image:ParallaxV2.svg|thumb|left|upright=1.36|Stellar parallax motion from annual parallax|alt=Diagrams illustrating the apparent change in position of a celestial object when viewed from different positions in Earth's orbit.]]
The parallax of a star is defined as half of the [[angular distance]] that a star appears to move relative to the [[celestial sphere]] as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the [[semimajor axis]] of the Earth's orbit. Substituting the star's parallax for the one arcsecond angle in the imaginary right triangle, the long leg of the triangle will measure the distance from the Sun to the star. A parsec can be defined as the length of the right triangle side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond.

The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the [[multiplicative inverse|reciprocal]] of the parallax angle in arcseconds (i.e.: if the parallax angle is 1&nbsp;arcsecond, the object is 1&nbsp;pc from the Sun; if the parallax angle is 0.5&nbsp;arcseconds, the object is 2&nbsp;pc away; etc.). No [[trigonometric function]]s are required in this relationship because the very small angles involved mean that the approximate solution of the [[skinny triangle]] can be applied.

Though it may have been used before, the term ''parsec'' was first mentioned in an astronomical publication in 1913. [[Astronomer Royal]] [[Frank Watson Dyson]] expressed his concern for the need of a name for that unit of distance. He proposed the name ''astron'', but mentioned that [[Carl Charlier]] had suggested ''[[siriometer]]'' and [[Herbert Hall Turner]] had proposed ''parsec''.<ref name=dyson /> It was Turner's proposal that stuck.

=== Calculating the value of a parsec ===

By the 2015 definition, {{Val|1|u=au}} of arc length subtends an angle of {{Val|1|u=arcsecond}} at the center of the circle of radius {{Val|1|u=pc}}. That is, 1 pc = 1 au/tan({{Val|1|u=arcsecond}}) ≈ 206,264.8 au by definition.<ref>{{cite journal|author=B. Luque|author2=F. J. Ballesteros| title=Title: To the Sun and beyond| date=2019|doi=10.1038/s41567-019-0685-3| journal=[[Nature Physics]]| volume=15|issue=12 | pages=1302|bibcode=2019NatPh..15.1302L |doi-access=free}}</ref> Converting from degree/minute/second units to [[radians]],

:<math>\frac{1 \text{ pc}}{1 \text{ au}} = \frac{180 \times 60 \times 60}{\pi}</math>, and
:<math>1 \text{ au} = 149\,597\,870\,700 \text{ m} </math> (exact by the 2012 definition of the au)

Therefore,
<math display="block">\pi ~ \mathrm{pc} = 180 \times 60 \times 60 ~ \mathrm{au} = 180 \times 60 \times 60 \times 149\,597\,870\,700 ~ \mathrm{m} = 96\,939\,420\,213\,600\,000 ~ \mathrm{m}</math> (exact by the 2015 definition)

Therefore,

<math display=block>1 ~ \mathrm{pc} = \frac{96\,939\,420\,213\,600\,000}{\pi} ~ \mathrm{m} = 30\,856\,775\,814\,913\,673 ~ \mathrm{m}</math> (to the nearest [[metre]]).

Approximately,

:[[Image:Parsec (1).svg|400px|Diagram of parsec.]]

In the diagram above (not to scale), '''S''' represents the Sun, and '''E''' the Earth at one point in its orbit (such as to form a right angle at '''S'''{{efn|name=orbit}}). Thus the distance '''ES''' is one astronomical unit (au). The angle '''SDE''' is one arcsecond ({{sfrac|3600}} of a [[degree (angle)|degree]]) so by definition '''D''' is a point in space at a distance of one parsec from the Sun. Through trigonometry, the distance '''SD''' is calculated as follows:

<math display=block>
\begin{align}
\mathrm{SD} &= \frac{\mathrm{ES} }{\tan 1''} \\
&= \frac{\mathrm{ES}}{\tan \left (\frac{1}{60 \times 60} \times \frac{\pi}{180} \right )} \\
&  \approx \frac{1 \, \mathrm{au} }{\frac{1}{60 \times 60} \times \frac{\pi}{180}} = \frac{648\,000}{\pi} \, \mathrm{au} \approx 206\,264.81 ~ \mathrm{au}.
\end{align}
</math>

Because the astronomical unit is defined to be {{Val|149597870700|ul=m}},<ref>{{Citation |title=Resolution B2 |date=31 August 2012 |contribution=Resolution B2 on the re-definition of the astronomical unit of length |contribution-url=http://www.iau.org/static/resolutions/IAU2012_English.pdf |place=Beijing |publisher=[[International Astronomical Union]] |quote=The XXVIII General Assembly of the International Astronomical Union recommends [adopted] that the astronomical unit be redefined to be a conventional unit of length equal to exactly {{Val|149597870700|u=m}}, in agreement with the value adopted in IAU 2009 Resolution B2}}</ref> the following can be calculated:

{| style="margin-left:1em"
|-
|rowspan=5 valign=top|Therefore, 1 parsec
|≈ {{Val|206264.806247096}} astronomical units
|-
|≈ {{Val|3.085677581|e=16}} metres
|-
|≈ {{Val|30.856775815}}&nbsp;trillion [[kilometre]]s
|-
|≈ {{Val|19.173511577}}&nbsp;trillion [[mile]]s
|}

Therefore, if {{Val|1|ul=ly}} ≈ {{Convert|1|ly|m|disp=out|sigfig=3}}, 
: Then {{Val|1|u=pc}} ≈ {{Val|3.261563777|u=ly}}

A corollary states that a parsec is also the distance from which a disc that is one au in diameter must be viewed for it to have an [[angular diameter]] of one arcsecond (by placing the observer at '''D''' and a disc spanning '''ES''').

Mathematically, to calculate distance, given obtained angular measurements from instruments in arcseconds, the formula would be:

<math display="block">\text{Distance}_\text{star} = \frac {\text{Distance}_\text{earth-sun}}{\tan{\frac{\theta}{3600}}}</math>

where ''θ'' is the measured angle in arcseconds, Distance<sub>earth-sun</sub> is a constant ({{Val|1|u=au}} or {{Convert|1|au|ly|disp=out|sigfig=5}}). The calculated stellar distance will be in the same measurement unit as used in Distance<sub>earth-sun</sub> (e.g. if Distance<sub>earth-sun</sub> = {{Val|1|u=au}}, unit for Distance<sub>star</sub> is in astronomical units; if Distance<sub>earth-sun</sub> = {{Convert|1|au|ly|disp=out|sigfig=5}}, unit for Distance<sub>star</sub> is in light-years).

The length of the parsec used in [[IAU]] 2015 Resolution B2<ref>{{Citation |title=Resolution B2 |date=13 August 2015 |contribution=Resolution B2 on recommended zero points for the absolute and apparent bolometric magnitude scales |contribution-url=http://www.iau.org/static/resolutions/IAU2015_English.pdf |place=Honolulu |publisher=[[International Astronomical Union]] |quote=The XXIX General Assembly of the International Astronomical Union notes [4] that the parsec is defined as exactly (648 000/<math>\pi</math>) au per the AU definition in IAU 2012 Resolution B2}}</ref> (exactly {{sfrac|{{Val|648000}}|{{pi}}}} astronomical units) corresponds exactly to that derived using the small-angle calculation. This differs from the classic inverse-[[tangent]] definition by about {{Val|200|u=km}}, i.e.: only after the 11th [[significant figure]]. As the astronomical unit was defined by the IAU (2012) as an exact length in metres, so now the parsec corresponds to an exact length in metres. To the nearest meter, the small-angle parsec corresponds to {{Val|30856775814913673|u=m}}.

== Usage and measurement ==
The parallax method is the fundamental calibration step for [[cosmic distance ladder|distance determination in astrophysics]]; however, the accuracy of ground-based [[telescope]] measurements of parallax angle is limited to about {{Val|0.01|u=arcsecond}}, and thus to stars no more than {{Val|100|u=pc}} distant.<ref>{{Cite web |title=Astronomy 162 |url=http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/distances.html |last=Pogge |first=Richard |publisher=Ohio State University}}</ref> This is because the Earth's atmosphere limits the sharpness of a star's image.{{cn|date=August 2022}} Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations. Between 1989 and 1993, the ''[[Hipparcos]]'' satellite, launched by the [[European Space Agency]] (ESA), measured parallaxes for about {{Val|100000}} stars with an [[astrometry|astrometric]] precision of about {{Val|0.97|ul=mas}}, and obtained accurate measurements for stellar distances of stars up to {{Val|1000|u=pc}} away.<ref>{{Cite web |title=The Hipparcos Space Astrometry Mission |url=http://www.rssd.esa.int/index.php?project=HIPPARCOS |access-date=28 August 2007}}</ref><ref>{{Cite web |title=From Hipparchus to Hipparcos |url=http://wwwhip.obspm.fr/hipparcos/SandT/hip-SandT.html |last=Turon |first=Catherine}}</ref>

<!-- [[NASA]]'s [[Full-sky Astrometric Mapping Explorer|''FAME'' satellite]] was to have been launched in 2004, to measure parallaxes for about 40&nbsp;million stars with sufficient precision to measure stellar distances of up to 2000&nbsp;pc. However, the mission's funding was withdrawn by NASA in January 2002.<ref>[http://www.usno.navy.mil/FAME/news/ FAME news], 25 January 2002.</ref> -->
ESA's [[Gaia mission|''Gaia'' satellite]], which launched on 19 December 2013, is intended to measure one billion stellar distances to within {{Val|20|u=microarcsecond}}s, producing errors of 10% in measurements as far as the [[Galactic Center|Galactic Centre]], about {{Val|8000|u=pc}} away in the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]].<ref>{{Cite web |title=GAIA |url=http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26 |publisher=[[European Space Agency]]}}</ref>

== Distances in parsecs ==

=== Distances less than a parsec ===
Distances expressed in fractions of a parsec usually involve objects within a single star system. So, for example:
* One astronomical unit (au), the distance from the Sun to the Earth, is just under {{Val|5|e=-6|u=parsec}}.
* The most distant [[space probe]], ''[[Voyager 1]]'', was {{Val|0.0007897|u=parsec}} from Earth {{As of|2024|February|lc=on}}. ''Voyager 1'' took {{Val|46|u=years}} to cover that distance.
* The [[Oort cloud]] is estimated to be approximately {{Val|0.6|u=parsec}} in [[diameter]]

[[Image:M87 jet.jpg|upright|thumb|As observed by the [[Hubble Space Telescope]], the [[astrophysical jet]] erupting from the [[active galactic nucleus]] of [[Messier 87|M87]] [[subtends]] {{Val|20|u=arcsecond}} and is thought to be {{Convert|1.5|kpc|ly|lk=out|sigfig=4}} long (the jet is somewhat foreshortened from Earth's perspective).]]

=== Parsecs and kiloparsecs ===
Distances expressed in parsecs (pc) include distances between nearby stars, such as those in the same [[spiral arm]] or [[globular cluster]]. A distance of {{Convert|1000|pc|ly|sigfig=4}} is denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to express distances between parts of a [[galaxy]] or within [[galaxy group|groups of galaxies]].<ref>{{Cite web |author1=Andrew May |date=2022-07-29 |title=What is a parsec? Definition and calculation |url=https://www.space.com/parsec |access-date=2025-01-16 |website=Space.com |language=en}}</ref> So, for example :
* [[Proxima Centauri]], the nearest known star to Earth other than the Sun, is about {{Convert|1.3|pc|ly|sigfig=3}} away by direct parallax measurement.<ref>{{Cite web |title=How Do We Know How Far Away the Stars Are? |url=https://www.britannica.com/story/how-do-we-know-how-far-away-the-stars-are#:~:text=The%20closest%20star,%20Proxima%20Centauri,would%20take%20950%20million%20years. |access-date=2025-01-16 |website=www.britannica.com |language=en}}</ref>
* The distance to the [[open cluster]] [[Pleiades]] is {{Val|130|10|u=pc}} ({{Val|420|30|u=ly}}) from us per ''[[Hipparcos]]'' parallax measurement.<ref>{{Cite web |title=The Pleiades Star Cluster {{!}} Royal Observatory Greenwich Astronomy Guides |url=https://www.rmg.co.uk/stories/topics/what-are-pleiades#:~:text=The%20Pleiades%20(pronounced%20'Ply-,light%20years%20away%20from%20Earth. |access-date=2025-01-16 |website=www.rmg.co.uk |language=en}}</ref>
* The [[Galactic Center|centre]] of the [[Milky Way]] is more than {{Convert|8|kpc|ly}} from the Earth and the Milky Way is roughly {{Convert|34|kpc|ly}} across.<ref>{{Cite web |date=2018-01-10 |title=Scientists Take Viewers to the Center of the Milky Way - NASA |url=https://www.nasa.gov/universe/scientists-take-viewers-to-the-center-of-the-milky-way/#:~:text=The%20Earth%20is%20located%20about,the%20center%20of%20the%20Galaxy. |access-date=2025-01-16 |language=en-US}}</ref>
* [[ESO 383-76]], one of the [[List of largest galaxies|largest known galaxies]], has a diameter of {{Convert|540.9|kpc|e6ly|1|abbr=unit}}.<ref>{{Cite web |date=18 September 2022 |title=Eso 383-76 Galaxy Facts, Distance & Size |url=https://www.universeguide.com/galaxy/eso-383-76 |access-date=2025-01-16 |website=Universe Guide |language=en-us}}</ref>
* The [[Andromeda Galaxy]] ([[Messier object|M31]]) is about {{Convert|780|kpc|e6ly|abbr=unit}} away from the Earth.<ref>{{Cite web |title=The Galaxy Next Door |url=https://www.nasa.gov/image-article/galaxy-next-door/#:~:text=At%20approximately%202.5%20million%20light,spans%20260,000%20light-years%20across. |access-date=2025-01-16 |language=en-US}}</ref>

=== Megaparsecs and gigaparsecs ===
<!-- Template:Convert/Mpc & Template:Convert/Gpc link here. -->
Astronomers typically express the distances between neighbouring galaxies and [[galaxy cluster]]s in megaparsecs (Mpc). A megaparsec is one million parsecs, or about 3,260,000 light years.<ref>{{cite web |url=https://astronomy.com/magazine/ask-astro/2020/02/why-is-a-parsec-326-light-years |title=Why is a parsec 3.26 light-years? |website=Astronomy.com |date=1 February 2020 |access-date=20 July 2021 }}</ref> Sometimes, galactic distances are given in units of Mpc/''h'' (as in "50/''h''&nbsp;Mpc", also written "{{nowrap|50 Mpc ''h''<sup>−1</sup>}}"). ''h'' is a constant (the "[[dimensionless Hubble constant]]") in the range {{nowrap|0.5 < ''h'' < 0.75}} reflecting the uncertainty in the value of the [[Hubble constant]] ''H'' for the rate of expansion of the universe: {{nowrap|1=''h'' = {{sfrac|''H''|100&nbsp;(km/s)/Mpc}}}}. The Hubble constant becomes relevant when converting an observed [[redshift]] ''z'' into a distance ''d'' using the formula {{nowrap|''d'' ≈ {{sfrac|''[[Speed of light|c]]''|''H''}} × ''z''}}.<ref>{{Cite web |title=Galaxy structures: the large scale structure of the nearby universe |url=http://pil.phys.uniroma1.it/twiki/bin/view/Pil/GalaxyStructures |url-status=dead |archive-url=https://web.archive.org/web/20070305202144/http://pil.phys.uniroma1.it/twiki/bin/view/Pil/GalaxyStructures |archive-date=5 March 2007 |access-date=22 May 2007}}</ref>

One gigaparsec (Gpc) is [[1000000000 (number)|one billion]] parsecs — one of the largest [[Orders of magnitude (length)|units of length]] commonly used. One gigaparsec is about {{Convert|1|Gpc|e9ly|sigfig=3|abbr=unit|disp=out}}, or roughly {{sfrac|14}} of the distance to the [[Cosmological horizon#Practical horizons|horizon]] of the [[observable universe]] (dictated by the [[cosmic microwave background radiation]]). Astronomers typically use gigaparsecs to express the sizes of [[Large-scale structure of the cosmos|large-scale structures]] such as the size of, and distance to, the [[CfA2 Great Wall]]; the distances between galaxy clusters; and the distance to [[quasar]]s.

For example:
* The [[Andromeda Galaxy]] is about {{Convert|0.78|Mpc|e6ly|abbr=unit}} from the Earth.
* The nearest large [[galaxy cluster]], the [[Virgo Cluster]], is about {{Convert|16.5|Mpc|e6ly|abbr=unit}} from the Earth.<ref>{{Cite journal |last1=Mei |first1=S. |last2=Blakeslee |first2=J.&nbsp;P. |last3=Côté |first3=P. |display-authors=etal |date=2007 |title=The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-dimensional Structure of the Virgo Cluster |journal=The Astrophysical Journal |volume=655 |issue=1 |pages=144–162 |arxiv=astro-ph/0702510 |bibcode=2007ApJ...655..144M |doi=10.1086/509598|s2cid=16483538 }}</ref>
* The galaxy [[RXJ1242-11]], observed to have a [[supermassive black hole]] core similar to the [[Milky Way]]'s, is about {{Convert|200|Mpc|e6ly|abbr=unit}} from the Earth.
* The [[galaxy filament]] [[Hercules–Corona Borealis Great Wall]], currently{{when?|date=January 2025}} the [[List of largest known cosmic structures|largest known structure]] in the universe, is about {{Convert|3|Gpc|e9ly|abbr=unit}} across.
* The [[particle horizon]] (the boundary of the [[observable universe]]) has a radius of about {{Convert|14|Gpc|e9ly|abbr=unit}}.<ref>{{Cite journal |last1=Lineweaver |first1=Charles H. |last2=Davis |first2=Tamara M. |date=1 March 2005 |title=Misconceptions about the Big Bang |url=http://www.scientificamerican.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 |url-status=dead |journal=Scientific American |volume=292 |issue=3 |pages=36–45 |bibcode=2005SciAm.292c..36L |doi=10.1038/scientificamerican0305-36 |archive-url=https://web.archive.org/web/20110810231727/http://www.scientificamerican.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 |archive-date=10 August 2011 |access-date=4 February 2016}}</ref>

== Volume units ==
To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs{{efn|name=vol|{{aligned table
|{{Val|1|u=pc3}}|[[Approximation|≈]] {{Val|2.938|e=49|u=m3}}
|{{Val|1|u=kpc3}}|≈ {{Val|2.938|e=58|u=m3}}
|{{Val|1|u=Mpc3}}|≈ {{Val|2.938|e=67|u=m3}}
|{{Val|1|u=Gpc3}}|≈ {{Val|2.938|e=76|u=m3}}
|{{Val|1|u=Tpc<sup>3</sup>}}|≈ {{Val|2.938|e=85|u=m3}}
}}}} (kpc<sup>3</sup>) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds, and interstellar gas is determined in a similar fashion. To determine the number of galaxies in [[supercluster]]s, volumes in cubic megaparsecs{{efn|name=vol}} (Mpc<sup>3</sup>) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge [[Boötes void]] is measured in cubic megaparsecs.<ref name="KirshnerOemler1981">{{Cite journal |last1=Kirshner |first1=R.&nbsp;P. |last2=Oemler | first2=A. Jr. |last3=Schechter |first3=P.&nbsp;L. |last4=Shectman |first4=S.&nbsp;A. |year=1981 |title=A million cubic megaparsec void in Bootes |journal=The Astrophysical Journal |volume=248 |pages=L57 |bibcode=1981ApJ...248L..57K |doi=10.1086/183623 |issn=0004-637X}}</ref>

In [[physical cosmology]], volumes of cubic gigaparsecs{{efn|name=vol}} (Gpc<sup>3</sup>) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars. The Sun is currently the only star in its cubic parsec,{{efn|name=vol}} (pc<sup>3</sup>) but in globular clusters the stellar density could be from {{Val|100|-|1000|u=pc<sup>−3</sup>}}.

The observational volume of gravitational wave interferometers (e.g., [[LIGO]], [[Virgo interferometer|Virgo]]) is stated in terms of cubic megaparsecs{{efn|name=vol}} (Mpc<sup>3</sup>) and is essentially the value of the effective distance cubed.

==See also==
* [[List of humorous units of measurement#Attoparsec|Attoparsec]]
* [[Distance measure]]

==In popular culture==
	
The parsec was used incorrectly as a measurement of time by [[Han Solo]] in the first ''[[A New Hope|Star Wars]]'' film, when he claimed his ship, the ''[[Millennium Falcon]]'' "made the Kessel Run in less than 12 parsecs", originally with the intention of presenting Solo as "something of a bull artist who didn't always know precisely what he was talking about". The claim was repeated in ''[[Star Wars: The Force Awakens|The Force Awakens]]'', but this was [[retcon]]ned in ''[[Solo: A Star Wars Story]]'', by stating the ''[[Millennium Falcon]]'' traveled a shorter distance (as opposed to a quicker time) due to a more dangerous route through the Kessel Run, enabled by its speed and maneuverability.<ref>{{Cite web |date=30 May 2018 |title='Solo' Corrected One of the Most Infamous 'Star Wars' Plot Holes |url=https://www.esquire.com/entertainment/movies/a20967903/solo-star-wars-kessel-distance-plot-hole/ |website=Esquire}}</ref> It is also used incorrectly in ''[[The Mandalorian]]''.<ref>{{Cite web |last=Choi |first=Charlse |date=5 November 2019 |title='Star Wars' Gets the Parsec Wrong Again in 'The Mandalorian' |url=https://www.space.com/star-wars-the-mandalorian-parsec.html |access-date=6 May 2020 |website=space.com}}</ref>

==Notes==
{{notes}}

==References==
{{reflist}}

== External links ==
* {{Cite web |title=Astronomical Distance Scales |url=http://csep10.phys.utk.edu/guidry/violence/distances.html |last=Guidry |first=Michael |website=Astronomy 162: Stars, Galaxies, and Cosmology |publisher=University of Tennessee, Knoxville |url-status=dead |archive-url=https://archive.today/20121212134512/http://csep10.phys.utk.edu/guidry/violence/distances.html |archive-date=12 December 2012 |access-date=26 March 2010}}
* {{Cite web |title=pc Parsec |url=http://www.sixtysymbols.com/videos/parsec.htm |last=Merrifield |first=Michael |website=Sixty Symbols |publisher=[[Brady Haran]] for the [[University of Nottingham]]}}

{{Units of length used in Astronomy}}
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[[Category:Units of length]]
[[Category:Units of measurement in astronomy]]
[[Category:Concepts in astronomy]]
[[Category:Parallax]]
[[Category:1913 in science]]